A. Air-Sparged Hydrocyclones
Flotation systems are important unit operations in process engineering technology that were developed to separate particulate constituents from slurries. Flotation is a process whereby air is bubbled through a suspension of finely dispersed particles, and the hydrophobic particles are separated from the remaining slurry by attachment to the air bubbles. The air bubble/particle aggregate, formed by adhesion of the bubble to the hydrophobic particles, is generally less dense than the slurry, thus causing the aggregate to rise to the surface of the flotation vessel. Separation of the hydrophobic particles is therefore accomplished by separating the upper layer of the slurry which is in the form of a froth or foam, from the remaining liquid.
The fundamental step in froth flotation involves air bubble/particle contact for a sufficient time to allow the particle to rupture the air-liquid film and thus establish attachment. The total time required for this process is the sum of contact time and induction time, where contact time is dependent on bubble/particle motion and on the hydrodynamics of the system, whereas induction time is controlled by the surface chemistry properties of the bubble and particle.
However, flotation separation has certain limitations that render it inefficient in many applications. Particularly, flotation is not very effective for the recovery of fine particles (less than 10 microns in diameter). This can be a serious limitation, especially in the separation of fine minerals. An explanation for this low recovery is that the particle's inertia is so small that particle penetration of the air bubble film is inhibited, thus resulting in low rates of attachment to the bubbles.
A further limitation of conventional flotation systems is that nominal retention times in the order of several minutes are required to achieve successful separation. However, it has been shown that air bubble/particle attachment is frequently in the order of milliseconds, therefore indicating that the rate of separation is mostly limited by bubble-to-particle collisions and/or transport rather than by other factors. As such, these long retention times severely limit plant capacity and require the construction of relatively large and expensive equipment.
Air-sparged hydrocyclones (hereinafter "ASH") were developed to overcome these two limitations of conventional flotation systems. U.S. Pat. Nos. 4,279,743, 4,397,741, 4,399,027 and 4,744,890 disclose the conventional ASH and certain improvements thereon. ASHs combine flotation separation principles with centrifugal forces to achieve successful separation of finer particles with retention times in the order of several seconds. A controlled high force field is established in the ASH by causing the slurry to flow in a swirling fashion, thereby increasing the inertia of the finer particles. Also, high density, small diameter air bubbles are forced through the slurry to increase collision rates with the particles. The net result is flotation rates with retention times approaching intrinsic bubble attachment times. This corresponds to a capacity that is at least 100 to 300 times the capacity of a conventional mechanical or column flotation unit.
In ASH flotation, fluid pressure energy is used to create rotational fluid motion (swirling motion). This is done by feeding the slurry tangentially through a conventional cyclone header into a cylindrical vessel. A swirl flow of a certain thickness is developed in the circumferential direction along the vessel wall, and is discharged through an annular opening created between the vessel wall and a pedestal located axially on the vessel's bottom.
Air is introduced into the ASH through the jacketed porous vessel walls, and is sheared into numerous small bubbles by the high velocity swirl flow of the slurry. Hydrophobic particles in the slurry collide with the air bubbles, attach to the bubbles, and are transported radially by the bubbles into a froth phase that forms in the cylindrical axis. The froth phase is supported and constrained by the pedestal at the bottom of the vessel, thus forcing the froth to move upward towards the vortex finder of the cyclone header, and to be discharged as an overflow product. The hydrophilic particles, on the other hand, generally remain in the slurry phase, and thus continue to move in a swirling direction along the porous vessel wall until they are discharged with the slurry phase through the annulus opening between the vessel wall and the pedestal.
It is important to note that the swirling motion of the slurry along the vessel wall forms a "swirl-layer" that is distinguishable from the forth phase at the center of the cylindrical vessel. One important characteristic of the swirl-layer is that it has a net axial velocity toward the underflow discharge annulus between the vessel wall and the froth pedestal. The thickness of the swirl-layer is generally 8% to 12% of the vessel radius, and it increases with increasing air flow rate and with axial distance from the cyclone header, being greatest at the underflow discharge annulus.
The size and motion features of the froth formed along the cylindrical vessel's axis are dependent on operating conditions and feed characteristics. Between the swirl-layer and the froth core, there exists a transition region for the slurry, where the net velocity in the axial direction is either zero, or in the same direction as the slurry phase. The latter condition exists where the forth core is relatively small, thus leaving a large gap between the swirl-layer and the froth core track is filled with slurry. The most desirable condition is when the transition region is minimal, that is when the froth core is large enough to leave little space between it and the swirl-layer.
A pressure drop is created in the froth core, between the froth pedestal and the vortex finder outlet located axially at the top of the vessel. This pressure drop is the force that actually drives the froth axially upwards. There are three factors that affect the pressure drop in the forth core:
1. restriction of the slurry flow to the underflow discharge annulus; PA1 2. restriction of the froth transport to the overflow vortex finder opening; and PA1 3. continuous supply of fresh froth to the froth core from the swirl-layer. PA1 F.sub.m is the magnetic force PA1 V.sub.p is the volume PA1 J.sub.p is the magnetic polarization of the particle PA1 .gradient.B.sub.o is the gradient of the external magnetic field PA1 .mu..sub.o is the permeability of the medium. PA1 D is the demagnetizing factor of the particle, and is O&lt;D&lt;1; and PA1 B is the magnetic flux density. PA1 (i) the particles themselves (size, magnetic susceptibility, density); PA1 (ii) the retention time of separating forces acting on particles; PA1 (iii) the magnitude and geometry of the non-uniform magnetic field; and PA1 (iv) the geometry of magnetic and non-magnetic discharge posts. PA1 1. effecting suspension and dispersion of small particles to prevent sedimentation and to permit contacting with air bubbles; PA1 2. influx of air, bubble formation, and bubble dispersion; PA1 3. conditions favoring particle bubble contact and attachment; PA1 4. a non-turbulent surface region for stable froth formation and removal; and PA1 5. in some cases sufficient mixing for further mineral reagent interaction. PA1 PULP: Bubble genecies; particle/bubble relative flow path; thinning and rapture of separating liquid films; highly aerated impeller region and less aerated reminder with intense recycle flow between two regions; steep pulp velocity gradients especially in the presence of frothing agent; distribution of residence time of solids. PA1 FROTH: Concentration gradients arising from selective and clinging action of froth column; bubble coalescence; concentration gradients may be represented by layering with step-wise concentration changes and two way mass transfer between the layers. PA1 PULP-FROTH TRANSITION: Two-way solid and liquid mass transfer between phases. PA1 AIR: Proves the motive force for both solids and water transfer from pulp to froth. PA1 WATER: Transported by air and all solids non-selectively at increasing rate with decreasing particle size, into froth column, aids return of solids from froth and pulp by drainage.
Factors 1 and 2 are in turn dependent on the particular application and can be adjusted during the operation. Factor 3 is dependent on air flow rate and on the hydrophobic properties of the particles, and their weight fraction in the feed slurry.
An immediate advantage of the ASH is the directed motion and intimate contact between the particles in the swirl-layer on the porous vessel wall and the freshly formed air bubbles. The high centrifugal force field developed by the swirling slurry imparts more inertia to the fine particles so that they can impact the bubble surface and attach to the bubbles. As a result, separation of fine particles is enhanced.
However, ASHs are relatively poor separators of coarser hydrophobic particles because the velocity of the swirling slurry imparts too high an inertia to these particles, thus preventing these particles from attaching to the air bubbles. As such, to achieve separation of these coarser particles, it is necessary that they exhibit relatively strong hydrophobicity so that the bubble/particle aggregate are stable under the prevailing hydrocyclone conditions. In cases where hydrophobicity is not strong enough, the system will exhibit some characteristics of a classification cyclone in that the coarse hydrophobic particles will be transported by the slurry to the underflow discharge annulus, while the finer particles will have a tendency to be transported into the froth core and out through the overflow vortex finder.
Studies have shown that the separation efficiency for a number of mineral particles falls as particle diameters increase above 100 microns. However, other studies show that the upper particle size limit is strongly affected by the hydrophobicity of the particle (as discussed above), and thus can be extended beyond 100 microns. For coal particles, testing shows that separations of particles above 100 to 400 microns drops significantly with increasing slurry pressure.
Therefore, an important addition to the art would occur if a method and apparatus is developed that can effectively separate particles of sizes beyond the present range of particle sizes. Also, a significant improvement would occur if increased slurry pressure (therefore increased feed flow rates) can be used while maintaining efficient separation.